Telephone systems and certain kinds of electrically powered control systems and security systems and certain kinds of electrically powered distribution systems are characterized in that many remotely positioned pieces of apparatus are interconnected with power sources and other apparatus at a central location. The telephone system is an example. Many remotely positioned subscriber telephones are interconnected with relay and switching apparatus at a central office. Interconnection is accomplished by attaching subscriber lines to switch gear, and that is accomplished by bridging terminals at a structure that is usually called a "main distribution frame."
When connection and disconnection is to be made between telephone circuit lines and apparatus that is disconnected from any electrical power source, the task of identification and connection is relatively simple. The task is made complicated, however, when the work must be done on a telephone system and frame that is energized and in operation. The problem is to avoid adversely effecting telephone calls in process. The problem is particularly severe when digital data is being transmitted through the system. In that case, the test activity must not introduce transient spikes that might look like data bits.
The magnitude that can be reached in the problem can be understood by envisioning the circumstance in which the main distribution frame of a telephone system is to be replaced with a new frame without interrupting normal operation of the system as energized, or "wet," lines are connected to the new frame and disconnected from the old. It appears that none of the continuity testers or wire identification apparatus of the prior art is capable of performing the task without producing an effect upon the system that might be disruptive of telephone calls in process, or that might result in unwanted circuit switching.
Just what the total effect of existing continuity testers and wire identification systems would be in that application is difficult to predict. Various telephone companies have adopted somewhat different standards. They employ different apparatus, and apparatus that is added to systems is often different in its electrical character from older apparatus in the same system. Voltages, impedance values, circuit routes are different from system to system, and within systems.
Essentially, the problem in identifying terminals and completing jumper connections involves placing one end of a continuity tester probe on one terminal or wire at one side of the main distribution frame and then looking for the corresponding terminal or wire on the other side of the frame by touching terminals and wires until the instrument indicates that there is continuity. The process ordinarily involves making many incorrect connections in order to find the correct one. That is, in fact, one preferred mode of operation. The task of interconnection is greatly simplified if the wire person can connect one side of the continuity tester to a terminal on one side of the frame and then run a connector probe, picket-fence fashion, over the terminals at the opposite side to identify at which terminals there is continuity.
A continuity tester must include a voltage source so that it can identify continuity in otherwise unpowered circuits. The problem is how to create a continuity tester, and how to conduct a continuity testing process in a way that will preclude testing voltage source from adversely effecting the operating telephone system, and in a way that will prevent the power sources of the system from causing erroneous test results.